Walruses feed on clams and other bottom-dwelling creatures in the shallower waters along the coasts and usually rest on ice floes between meals, said Bruce Woods, a spokesperson for the U.S. Fish and Wildlife Service in Anchorage, Alaska.

"They came ashore in places that we hadn't seen before and in numbers that in many cases were considerably larger than we'd ever seen before," Woods said.

The crowding was particularly acute on the Russian side of the Bering Strait, according to Russian biologists who collaborate with the U.S. Fish and Wildlife Service.

At one location, the scientists counted 30,000 walruses.

Like any herd animal, walruses are easily spooked, Woods noted. Loud boats, low-flying airplanes, or the sight of predators such as polar bears cause them to panic and rush toward the water.

"Particularly when their numbers are so great, sometimes walruses are hurt or killed in stampedes, and the ones that tend to be hurt or killed, of course, are going to be the smaller animals," Woods said.

Russian biologists counted about 3,000 to 4,000 walrus carcasses along the coast in the areas they surveyed—higher than normal. Polar bears were spotted scavenging the remains.